Plasma-etching and/or reactive ion etching of silicon substrates can result in etch residues. For example, plasma-etching and/or reactive ion etching of metals or metal alloys on silicon substrates can create etch residues containing metal. These residues are undesirable, as they impair the resolution of desirable features on the silicon substrate and shorten device life. Therefore, it is desirable to remove these residues.
When etching layers on a silicon substrate, distinctly different etch residues are created as the plasma or reactive ion etch attacks each film or layer in the silicon substrate in succession. Therefore, most etch residues resulting from plasma or reactive ion etching are composed of distinctly different residues. Unfortunately, most solutions used to remove these etch residues suffer from a lack of selectivity. For example, most solutions attack some residues less aggressively than others. However, if longer dip times are employed to remove the more difficult residues, these solutions begin to attack other features on the substrate.
In an effort to increase selectivity and control, various solutions for removing etch residues have been developed. For example, U.S. Pat. No. 5,320,709 to Bowden et al. teach the selective removal of etch residues with a polyhydric alcohol containing 0.5% to 10% anhydrous ammonium fluoride salt. Bowden et al. teach that the selectivity observed is due to the presence of little (&lt;4%) or no water. Bowden et al. teach that because there is little or no water present, essentially no H.sub.3 O.sup.+ is formed and the small amount of HF evolved does not ionize. Therefore, the solution is essentially neutral and hence, removal rates of the etch residues are slower. In addition, Bowden et al. teach that smaller amounts of ammonium fluoride yield slower etch residue removal rates, and larger amounts may be used but are limited by the solubility of ammonium fluoride in the polyhydric alcohol. Further, while Bowden et al. teach anhydrous solutions of ammonium fluoride in various polyhydric alcohols, they disclose only ethylene glycol solutions containing 4% anhydrous ammonium fluoride.
Due to environmental concerns, the use of ethylene glycol is problematic. Although higher order polyhydric alcohols do not share the environmental concerns associated with the use of ethylene glycol, the use of higher order polyhydric alcohols in solvents intended to remove etch residues is not straightforward. For example, anhydrous solutions of ammonium fluoride in higher order polyhydric alcohols less efficient in removing etch residues. Anhydrous solutions of ammonium fluoride in higher order polyhydric alcohols tend to exhibit slower etch residue removal rates when compared to the etch residue removal rates of anhydrous solutions of ammonium fluoride in ethylene glycol. These slower etch residue removal rates are due to the lower solubility of anhydrous ammonium fluoride in higher order polyhydric alcohols as compared with solutions of anhydrous ammonium fluoride in ethylene glycol. Because anhydrous ammonium fluoride is less soluble in anhydrous higher order polyhydric alcohols than in anhydrous ethylene glycol, anhydrous solutions of ammonium fluoride in higher order polyhydric alcohols necessarily contain less ammonium fluoride then do anhydrous solutions of ammonium fluoride in ethylene glycol. Hence, etch residue removal rates are slower and the higher order polyhydric alcoholic solutions are not as efficient as anhydrous solutions of ammonium fluoride in ethylene glycol in removing etch residues.
The problems of etch residue removal efficiency and etch residue removal rates associated with the use of anhydrous solutions of ammonium fluoride in higher order polyhydric alcohols can be overcome by the addition of water. For example, when comparing like-polyhydric alcoholic solutions, it is possible to obtain higher concentrations of ammonium fluoride in aqueous solutions than in anhydrous solutions due to the higher solubility of ammonium fluoride in such aqueous solutions. Hence, when comparing like-polyhydric alcoholic solutions, faster etch residue removal rates and etch residue removal efficiencies are possible with aqueous solutions of ammonium fluoride then with anhydrous solutions of ammonium fluoride.
Aqueous solutions of ammonium fluoride in polyhydric alcohols exhibit several other advantages over anhydrous solutions of ammonium fluoride in like-polyhydric alcohols. For example, anhydrous solutions of ammonium fluoride in polyhydric alcohols, including ethylene glycol, are relatively viscous. Therefore, the wetting capabilities of these anhydrous solutions are more limited. Wetting capabilities are directly related to the rate of initiation of the etch residue removal process. Since it is usually desirable to have the etch residue removal process initiate rapidly, it is advantageous for a solution intended to remove etch residues to exhibit increased wetting capabilities. In addition, because the anhydrous solutions are relatively viscous, removal of the polyhydric alcohol solution from the substrate after the etch residue removal process with a simple water rinse can be problematic. The addition of water to polyhydric alcoholic solutions of ammonium fluoride improves not only wetting capabilities and rinsability, but also increases bath life. For example, when comparing like-polyhydric alcoholic solutions, aqueous solutions of ammonium fluoride are less viscous then anhydrous solutions of ammonium fluoride. Therefore, when comparing like-polyhydric alcoholic solutions, aqueous solutions of ammonium fluoride exhibit greater wetting capabilities then do anhydrous solutions of ammonium fluoride. In addition, this lower viscosity of the etch solution leads to increased rinsability after the substrate is removed from the solution. Because aqueous solutions of ammonium fluoride in polyhydric alcohols are less viscous then anhydrous solutions (when compared with like polyhydric alcohols), less water or other rising fluid is required to displace the polyhydric alcohol from the exposed surface of the substrate. Further, because fluoride ion is consumed during the etching process, the higher concentrations of ammonium fluoride which may be obtained in aqueous solutions as compared with anhydrous solutions can result increased bath life.
Aqueous solutions of ammonium fluoride in higher order polyhydric alcohols are known. For example, U.S. Pat. No. 5,698,503 to Ward et al. teach acidic (pH&gt;4 but &lt;7) aqueous compositions for the controlled removal of etch residues comprising a polyhydric alcohol (excluding ethylene glycol), an organic water soluble polar solvent, and ammonium fluoride. In addition, Ward et al. teach that the use of an organic water soluble polar solvent allows use of ammonium fluoride in the formulation when propylene glycol is the polyhydric alcohol. In addition, U.S. Pat. No. 5,571,447 to Ward et al. discloses acidic (pH&lt;5) compositions for the controlled removal of etch residues comprising a polyhydric alcohol (excluding ethylene glycol), fluoboric acid (HBF.sub.4), and a small amount of fluoride salt, including ammonium fluoride, in polar solvents. Ward et al. teach that the fluoboric acid is an essential component of the composition in that it maintains a stable fluoride ion source with ammonium fluoride or other fluoride salts and provides the needed low pH of the composition. In addition, Ward et al. teach that the use of water or water/dimethylsulfoxide (DMSO) mixtures allows use of ammonium fluoride in the formulation when propylene glycol is the polyhydric alcohol. Ward et al. further teach that the compositions are water soluble, non-corrosive to the substrate, non-flammable and of low toxicity to the environment.
However, as taught by Bowden et al. in U.S. Pat. No. 5,320,709, acidic solutions of ammonium fluoride, like those taught by Ward et al. in U.S. Pat. Nos. 5,698,503 and 5,571,447 result in a loss of selectivity and control. Therefore, acidic solutions of ammonium fluoride in polyhydric alcohols, such as taught by Ward et al. in U.S. Pat. Nos. 5,698,503 and 5,571,447, are not compatible with certain features typically present on silicon substrates in the process of fabrication. For example, aluminum and dielectrics are generally attacked when exposed to acidic fluorinated solutions by the hydrogen fluoride (HF) which is present in such solutions. Therefore, when such susceptible features are present on the silicon substrate, it is often necessary that corrosion inhibitors, such as catechol and pyrogallol, be added to the acidic fluorinated solution to inhibit such attack. Very basic solutions can attack aluminum as well, and many basic chemistries also contain corrosion inhibitors.
In addition to the ammonium fluoride solutions of Bowden et al. and Ward et al. discussed above, other solutions of ammonium fluoride in polyhydric alcohols are known. However, these solutions are not known to be useful for removing etch residues from silicon substrates which result from plasma or reactive ion etching of silicon substrates. In addition, these solutions differ from those of the present invention in that they are anhydrous in nature, contain ethylene glycol, and/or have a pH of less than 7. For example, U.S. Pat. No. 3,979,241 to Maeda et al. discloses an anhydrous solution of ammonium fluoride in ethylene glycol useful for etching silicon dioxide and silicon nitride; U.S. Pat. No. 4,087,367 to Rioult et al. discloses an anhydrous solution of ammonium fluoride in ethylene glycol useful for selective removal of aluminum oxide; U.S. Pat. No. 4,165,295 to Vander Mey teaches solutions containing organic sulfonic acids in combination with fluoride ions, including ammonium fluoride, in organic solvents useful for stripping photoresists from metallized inorganic substrates; U.S. Pat. No. 4,343,677 to Kinsbron et al. discloses liquid compositions of ammonium fluoride and hydrofluoric acid useful for buildups of silicon oxides; and U.S. Pat. No. 5,376,236 to Hanson et al. discloses aqueous solutions of hydrofluoric acid and ammonium fluoride in propylene glycol useful for etching titanium. In addition, MICROSTRIP 5003, available from Olin, is an aqueous solutions containing ethylene glycol and ammonium fluoride and is known as being useful as an oxide etchant. Further, the NOE series of products, available from Allied Chemical Sciences, Inc., are anhydrous solutions of ammonium fluoride in ethylene glycol or propylene glycol and are known as being useful for etching oxides.
Therefore, there exists a need in the art for compositions which remove etch residues created during plasma-etching and/or reactive ion etching of silicon substrates which overcome the environmental concerns associated with the use of ethylene glycol and which are compatible with other features normally present on a silicon substrate.